The Paul Wellstone and Pete Domenici Mental Health Parity and Addiction Equity Act of 2008 (MHPAEA), which took effect for plans renewing from July 2010 on, significantly changed the law and regulations governing the design and management of mental health and substance abuse (MH/SA) benefits for private insurance products sold to employers with 50 or more employees. MHPAEA represents a sea change in the law, as its provisions go well beyond prior federal and state parity legislation. MHPAEA is a federal law, so it applies to self-insured plans that are exempt from state mandates, but unlike the previous federal parity law, MHPAEA includes substance use disorders, not just MH. MHPAEA also affects non-quantitative treatment limits (NQTLs) affecting utilization management (e.g., medical necessity review) in addition to benefit design features such as cost-sharing requirements and quantitative treatment limits (e.g., number of visits covered). The implications of this landmark piece of legislation are not yet known and are likely to be substantially greater than the effects of earlier laws. Our study would document the impact of the MHPAEA on MH/SA benefit design, management, utilization, costs and outcomes by analyzing unique administrative databases from the largest managed behavioral health organization in the country, OptumHealth Behavioral Solutions (OHBS). The study design will use pre-, transition and post-MHPAEA data to compare changes in study outcomes over time among plans, employer groups and patients expected to experience differential impact of the legislation. An additional comparison will be provided by a six-month difference in the timing of when employer groups had to comply with MHPAEA, based on their renewal periods. Aims 1-3, which are plan-level analyses, will examine the association of MHPAEA with changes in offer rates for MH/SA benefits, as well as changes in benefit designs and NQTLs among plans that continue to offer such benefits. These Aims will use detailed information on benefit design and NQTLs collected by OHBS on each health plan one year before and the first two years after parity. Aims 4-5, which are patient-level analyses, will examine the association of MHPAEA with changes in access, utilization and cost of MH/SA services and psychotropic drugs, using linked insurance claims and eligibility data from 2008 to 2013. All five Aims will examine how the changes in these endpoints vary by type of disorder (MH vs. SA), plan characteristics (e.g., small vs. large-group market, carve- in vs. carve-out), employer characteristics (firm size, whether self-insured), and pre-existing state parity laws (strong, weak or none). Aims 4 and 5 will also include patient characteristics. Findings from our evaluation will help to inform policymakers considering future modification to the MHPAEA or state legislation to supplement its provisions (e.g., mandates aimed at preventing employer groups from dropping coverage altogether); provide an evidence base for employer groups deciding whether to continue MH/SA coverage and which conditions to cover; and indicate promising avenues for future research.